The disclosure of Japanese Patent Application No. 2001-262011 filed on Aug. 30, 2001 including the specification, drawings, and abstract is incorporated herein by reference in its entirety.
1. Field of the Invention
This invention relates to a control apparatus of a lock-up clutch for a vehicle and a control method thereof, and more particularly to a control apparatus of a lock-up clutch for a vehicle which can extend a time for executing a fuel-cut control, and a control method thereof.
2. Description of the Related Art
There is a technique of increasing a fuel-cut time by slip controlling a lock-up clutch at a time when a vehicle is decelerated, thereby increasing a fuel consumption, and this technique is disclosed, for example, in Japanese Patent Application Laid-open Publication No. 11-257484. During a deceleration slip in which the lock-up clutch is slip controlled at a time when the vehicle is decelerated, since it is possible to maintain an engine rotational speed substantially at the same rotational speed as a turbine rotational speed, it is possible to maintain the engine rotational speed higher compared to a case of disengaging the lock-up clutch, and it is possible to increase the fuel-cut time.
However, in the technique mentioned above, at a time when the vehicle is decelerated in a high gear stage of an automatic transmission, since the fuel-cut control is finished earlier than a case of a low gear stage, there has been room for improvement in view of improving a fuel consumption. That is, a condition of executing the fuel-cut control has a lower limit due to the engine rotational speed. Further, the turbine rotational speed with respect to a vehicle speed is lower in the high gear stage than in the low gear stage. Even when the lock-up clutch is slip controlled, the engine rotational speed is only maintained at substantially the same rotational speed as the turbine rotational speed, so that there has been a problem that the fuel-cut time becomes shorter in the high gear stage than in the low gear stage.
Further, at the time of deceleration by initially setting to the low gear stage, the fuel-cut time is increased, however, there is a problem that an engine brake is applied too much, and an uncomfortable feeling is generated in a driver, so that it is hard to put it to practical use.
The reason why such a problem is generated is that since the turbine rotational speed becomes large and the engine rotational speed becomes large at a time of executing a shift down in a high speed vehicle, engine friction torque becomes large. Further, inertia torque is added accompanying with an increase of the engine rotational speed, and engine brake torque is amplified by an increased amount of gear ratio.
In order to solve the problems mentioned above, it is an object of the invention to provide a control apparatus of a lock-up clutch which can increase a time for executing a fuel-cut control, and a control method thereof.
In accordance with a first aspect of the invention, there is provided a control apparatus of a lock-up clutch for a vehicle. The control apparatus has fuel-cut control portion having a hydraulic power transmission with a lock-up clutch which directly connects between an engine and an automatic transmission, stopping supplying a fuel to the engine at a time when a speed of the vehicle is reduced, and restarting supplying the fuel to the engine at a time of judging that at least any one of the rotational speed of the engine and an input shaft rotational speed of the automatic transmission becomes a predetermined fuel-cut end rotational speed, a slip control portion which executes a slip control of the lock-up clutch at a time when the vehicle is decelerated, and a shift control portion which shifts down the automatic transmission at a time of judging that the rotational speed of the engine becomes a down shift rotational speed which is higher than the fuel-cut end rotational speed by a predetermined amount, during an execution of the slip control by the slip control portion.
In accordance with the first aspect mentioned above, at a time when at least any one of the rotational speed of the engine and the input shaft rotational speed of the automatic transmission becomes the rotational speed higher than the fuel-cut end rotational speed by the predetermined amount during the slip control of the lock-up clutch at a time when the vehicle is decelerated, the down shift of the automatic transmission is executed. Since an input shaft rotational speed of the automatic transmission becomes high in accordance with the down shift of the automatic transmission, it is possible to prevent the engine rotational speed from being reduced. As a result, it is possible to increase a time required until the engine rotational speed reaches the fuel-cut end rotational speed, and it is possible to increase a fuel-cut time. Further, since the down shift is executed immediately before the engine rotational speed reaches the fuel-cut end rotational speed, it is possible to restrict an adverse effect that an uncomfortable feeling is given to the driver due to the increase of the deceleration of the vehicle caused by an increase of an engine friction torque and an inertia torque.
Further, the slip control portion may change a slip amount of the lock-up clutch so as to restrict the increase of the engine rotational speed in accordance with the down shift of the automatic transmission.
Since the lock-up clutch is slipped so as to restrict the increase of the engine rotational speed caused by the down shift at a time of executing the down shift of the automatic transmission, it is possible to restrict the increase of the engine friction torque and the inertia torque, and it is possible to properly restrict the increase of the deceleration of the vehicle.
Further, the slip control portion may slip the lock-up clutch so as to prevent the rotational speed of the engine from being increased in accordance with the down shift of the automatic transmission.
Since the lock-up clutch is slipped so as to prevent the engine rotational speed from being increased, it is possible to properly restrict the increase of the engine friction torque and the inertia torque.
Further, the slip control portion may slip the lock-up clutch so as to keep the rotational speed of the engine during a period after the automatic transmission is shifted down and until the input shaft rotational speed of the automatic transmission reaches the predetermined rotational speed to the engine rotational speed immediately before the automatic transmission is shifted down.
Since the rotational speed of the engine during the period after the automatic transmission is shifted down and until the input shaft rotational speed of the automatic transmission reaches the predetermined rotational speed is kept to the engine rotational speed immediately before the automatic transmission is shifted down, it is possible to property restrict both of the increase of the inertia torque and the reduction of the engine rotational speed.
In accordance with a second aspect of the invention, there is provided a control apparatus of a lock-up clutch for a vehicle. The control apparatus has a slip control portion having a lock-up clutch which directly connects between an engine and an automatic transmission, and increasing a fuel-cut time required from a start of a fuel-cut at which a fuel supply to the engine is stopped and until an engine rotational speed reaches a fuel-cut end rotational speed, by slip controlling the lock-up clutch at a time when the vehicle is decelerated, a shift transmission control portion which increases the fuel-cut time by shifting down the automatic transmission immediately before the engine rotational speed reaches the fuel-cut end rotational speed.
In accordance with the second aspect, since the input shaft rotational speed of the automatic transmission becomes high by shifting down the automatic transmission immediately before the engine rotational speed reaches the fuel-cut end rotational speed, it is possible to restrict a reduction of the engine rotational speed. As a result, it is possible to increase the time required until the engine rotational speed reaches the fuel-cut end rotational speed, and it is possible to increase the fuel-cut time. Further, since the down shift is executed immediately before the engine rotational speed reaches the fuel-cut end rotational speed, it is possible to restrict the adverse effect that the uncomfortable feeling is given to the driver due to the increase of the deceleration of the vehicle in accordance with the increase of the engine friction torque and the inertia torque.
In accordance with a third aspect of the invention, there is provided a control method of a lock-up clutch for a vehicle having a hydraulic power transmission with a lockup clutch which directly connects between an engine and an automatic transmission, stopping supplying a fuel to the engine at a time when a speed of the vehicle is reduced, and restarting supplying the fuel to the engine at a time of judging that a rotational speed of the engine becomes a predetermined fuel-cut end rotational speed. The control method comprises executing a slip control of the lock-up clutch at a time when the vehicle is decelerated, shifting down the automatic transmission at a time of judging that at least any one of the rotational speed of the engine and the input shaft rotational speed of the automatic transmission becomes a down shift rotational speed which is higher than the fuel-cut end rotational speed by a predetermined amount, during an execution of the slip control by the slip control portion.
In accordance with a fourth aspect of the invention, there is provided a control method of a lock-up clutch for a vehicle having a lock-up clutch which directly connects between an engine and an automatic transmission. The control method comprises increasing a fuel-cut time required from a start of a fuel-cut at which a fuel supply to the engine is stopped and until an engine rotational speed reaches a fuel-cut end rotational speed, by slip controlling the lock-up clutch at a time when the vehicle is decelerated, and increasing the fuel-cut time by shifting down the automatic transmission immediately before the engine rotational speed reaches the fuel-cut end rotational speed.
In accordance with the third and fourth aspects, since the input shaft rotational speed of the automatic transmission becomes high by shifting down the automatic transmission, immediately before the engine rotational speed reaches the fuel-cut end rotational speed, it is possible to restrict the reduction of the engine rotational speed. As a result, it is possible to increase the time required until the engine rotational speed reaches the fuel-cut end rotational speed, and it is possible to increase the fuel-cut time.